Gasoline storage tank facilities, gasoline transport trucks, underground natural gas delivery systems, or other fluid storage or delivery systems generally have a positive displacement flow meter connected in line in the fluid delivery system. Pumping of the fluid, whether gas or liquid, through the delivery line causes movement of the rotors in the flow meter which drives a mechanical or electrical counting device to measure precisely the volume of fluid flow through the meter.
Some flow meters have a housing that defines a cavity within which three rotors are rotatably mounted. The three rotors include a pair of displacement rotors and a blocking rotor disposed between the displacement rotors. One of the displacement rotors is disposed towards the inlet of the flow meter; the other displacement rotor is disposed towards the outlet. As the blocking rotor rotates, it mates with the inlet displacement rotor disposed to close off part of the cavity to define a flow path along which the fluid must pass, thereby causing the displacement rotors and blocking rotor to rotate. The rotation of the displacement and blocking rotors creates a motion that can be correlated to the fluid volume passing through the meter, making it possible to translate the rotation of the displacement rotors into a meter reading showing fluid volume flow.
Typically, the entire fluid flow through a conduit is diverted through the flow meter in order to provide a flow throughput reading. Therefore, it is desirable that the flow meter add as little flow impedance as possible to the flow to minimize energy losses and to maintain the intended flow rate. To that end, it is desirable to provide a flow meter with rotors having low mass but sufficient strength for a long and accurate service life. For obvious reasons, it is also desirable to provide housings for flow meters that are as compact as possible.
A continuing need then exists to provide flow meters of the type utilizing blocking and displacement rotors of lightweight yet strong construction. Specifically, flow meters used for dispensing liquefied petroleum gas (LPG) are subjected to sudden focus in the event the LPG flashes. As a result, prior art blocking rotors are prone to breakage. Specifically, despite the use of reinforcing ribs 1400, the blocking rotor 115 of FIG. 2A is prone to breakage along the ribbed wall 140 near the spindle 155 while the blocking rotor 315 of FIG. 2C is prone to breakage on the wall 340 on either side of the shaft 355. Finally, the shaftless blocking rotor 215 of FIG. 2B is prone to breakage near the axial center of the wall 240.
Another problem with the prior art blocking rotors 115, 215, 315 of FIGS. 2A-2C is the high moments of inertia associated with each rotor 115, 215, 315 that require more energy to initiate rotation of the rotors 115, 215, 315 and more time to stop rotation of the rotors 115, 215, 315. The slower stopping ability of the rotors 115, 215, 315 in unwanted leakage through the flow meter after the dispensing has stopped thereby compromising the accuracy of the dispense.